1. Field of the Invention
The present invention relates to a terminal device and a method for use in a communication system in which a plurality of terminal devices communicate with each other, and more particularly, to a terminal device and a method for use in a communication system in which at least one of a plurality of terminal devices is a master station for controlling communication in the communication system.
2. Description of the Related Art
A communication system, where each terminal device uses a common communication medium, requires a communication protocol capable of realizing effective and satisfactory communication efficiency.
Conventionally, in the communication system, where each terminal device uses a common communication medium, a contention protocol is essentially used. A terminal device which uses the contention protocol verifies that no signal from another terminal device is present on the communication medium, and then starts signal transmission. However, if the number of terminal devices or network traffic is increased, there may be an increase in the number of terminal devices which simultaneously start signal transmission resulting in an increase of a packet collision rate. Thus, in the contention protocol network use efficiency is limited.
Accordingly, a centralized control protocol has been developed with the intention of promoting efficiency in an entire network by providing a master station for controlling access of a terminal device on the communication system to another terminal device. In the communication system using the centralized control protocol, a terminal device acting as the master station controls signal transmission and reception between terminal devices on the communication system. In the centralized control protocol, for example, only a terminal device to which transmission right is granted by the master station is allowed to transmit a signal.
However, in the case where communication quality on the communication medium varies among locations, the quality of communication between the master station and a slave station depends on a physical positional relationship between the master and the slave station. Accordingly, the physical positional relationship between the master station and the slave station has a considerable influence on communication efficiency on a communication network. Thus, there has been developed a centralized control protocol capable of optimizing the communication efficiency by selecting a master station, which is able to provide optimum communication quality to each slave station, from among a plurality of master stations whose physical location is fixed (see, for example, Japanese National Phase PCT Laid-Open Publication No. 2001-505035).
However, in a home network or the like, it is difficult to place the master station in a physically fixed location. In some cases, it is also difficult to place the master station in such a location as to optimize the communication efficiency. Accordingly, in the conventional centralized control protocol, the communication efficiency cannot necessarily be optimized.
Moreover, in the case where a communication system is provided adjacent to an external communication system using a common frequency band, signal interference occurs between the communication systems. Such interference by an external communication system is referred to as “external interference”. If the master station is not aware of the external interference, the master station is not able to control communication based on degradation of the communication quality due to the external interference. Particularly, if a power line is used as the communication medium, there may be cases where a signal leaks from, for example, an adjacent house, and therefore the external interference (interference between adjacent houses) is considerable.